Immediate meaning of DTF is Down To Fuck however it has become a 'call to arms' for a night out.

Friday night: Jimmy - Lads who is DTF tonight.
Sean - Sorry guys I have sand in my vigina and can't come out (hence not DTF)
Everyone else - I am way DTF (displaying an extreme level of DTF, hence is willing to go out, drink alcohol and try to pick up birds)